A critical step in semiconductor processing is photolithography. The ability to achieve smaller and smaller dimensions on an integrated circuit is generally understood to be limited more by photolithography than any other fabrication step of the semiconductor process. As the industry heads toward forming submicron line dimensions, advanced photolithographic capabilities become ever more critical. Various new photolithographic techniques using smaller wavelength light sources are being developed, including Deep Ultra Violet Lithography, Extreme Ultra Violet Lithography (EUVL), and x-ray lithography.
In typical photolithography techniques, radiation from a light source is projected through a reticle, that is, a patterned mask, and an image of the pattern on the reticle is focused through a lens onto the radiation sensitive photoresist layer of a semiconductor substrate. The substrate may be a silicon wafer or other semiconductor substrate on which integrated circuits or micromechanical structures are fabricated.
A typical reticle comprises a patterned opaque material applied to one side of a transparent base. The base, typically comprised of quartz, is transparent to the projected radiation. The patterned opaque material, typically chrome, is opaque to the projected radiation. In addition to the desired pattern any defect in the reticle will also be projected onto the photoresist layer of the semiconductor substrate. For example, if a particle is present on the reticle during exposure of the photoresist layer, the image of the particle may be focused onto the photoresist layer. This corresponding defect in the photoresist pattern on the semiconductor substrate may cause the failure of the semiconductor device being manufactured.
EUVL, which typically uses a light source with a wavelength on the order of 13 nanometers (nm), is a promising technology for submicron integrated circuit fabrication. The base of a typical reticle is not transparent to ultra violet radiation in the extreme ultra violet (EUV) range because of the strong absorption of the base material. Therefore, a reflective reticle is used in EUVL.
Even when using reflective reticles, any reticle defects may be imaged onto the photoresist layer of the semiconductor substrate. The surface of the reflective reticle is very difficult to keep clean and any semiconductor device being manufactured may fail if particles are present on the reticle. The images of the particles may be focused onto the photoresist layer during exposure, leading to unacceptable defects in the semiconductor device.
Typically, a pellicle is used to protect a reticle and to keep it clean. A pellicle is a thin, flat, transparent membrane, usually made of an organic material. The pellicle is held by a frame and placed over the reticle. The frame of the pellicle holds it several millimeters away from the patterned surface of the reticle. The pellicle keeps particles from falling onto the surface of the reticle. Any particles that fall onto the pellicle will be outside the focal plane of the photolithography system, and therefore will not focus onto the semiconductor wafer during exposure.
FIG. 1 illustrates a prior art photolithography mask having a pellicle 110 mounted on the surface of a reticle 100. Pellicle 110 forms a covered or protected area 120 over the patterned area of reticle 100. Pellicle 110 includes a frame 130 and a thin, transparent membrane 140.
Pellicle 110 is effective at reducing the likelihood that particles will migrate onto reticle 100; however, prior art organic pellicle membranes 140 cannot be used in EUVL because these pellicle membranes are not transparent to EUV radiation. The pellicle membranes absorb an unacceptable amount of ultra violet light in the EUV range, especially when using the reflective reticles required in EUVL. This is because the source EUV radiation is absorbed twice as it makes a dual pass through the pellicle membrane on its reflected path to the semiconductor wafer.
There exists a need for a non-pellicle device to protect reticles and to prevent defects in semiconductor devices manufactured using EUVL. It would be advantageous to have a reticle-protective device that is readily manufacturable and compatible with both EUVL and currently used photolithographic manufacturing techniques.